1. Field of the Invention
The invention relates to methods for the minimization or prevention of the formation of salt deposits in oil and gas wells. In particular, the use of nitrilotriacetamide in treating wells is disclosed.
2. Description of Related Art
Wells drilled to produce petroleum or gas products typically also produce brine water. The water is often near or at sodium chloride saturation levels. During handling of the water, salt crystals often form, producing a “salt block” in the well and/or flowlines. This block is typically formed by either temperature cooling of the water, or by concentration of the brine downhole as produced gas gradually strips water vapor leaving salt behind as an increasingly concentrated solution.
Salt blocks and deposits have historically been treated by circulation of fresh water into the well. While usually effective at dissolving the salt, this treatment can be expensive, inconvenient, and delays production. Water may not be readily accessible at the drilling site, and it may be necessary to transport it over potentially long distances. During water treatment, the production of the well is significantly reduced or stopped completely.
Salt inhibitors typically function by binding to the surfaces of salt crystals, preventing further growth of the crystal. This effectively increases the solubility of the salt in solution, as it is incapable of precipitating. Other mechanisms of action may be possible, resulting in an increased solubility of salt in solution. Various salt inhibitors have been discussed in the engineering literature.
SPE 10097 (Society of Petroleum Engineers of AIME, Oct. 5–7, 1961) describes the evaluation of five commercially available salt inhibitors. Laboratory assays were performed to determine effectiveness in maintaining salt saturation at downhole temperatures. Tests were also performed in a Williston Basin drilling program. The chemical compositions of the inhibitors were not disclosed.
U.S. Pat. No. 3,213,018 (issued Oct. 19, 1965) suggested the use of iron or cobalt salts as a method to inhibit deposition of sodium chloride from saturated solutions. A field trial using sodium ferrocyanide in an oil well was reported.
U.S. Pat. No. 3,367,416 (issued Feb. 6, 1968) offered the use of nitrilotriacetamide or its acid salts to reduce deposition of sodium chloride from supersaturated solutions undergoing reductions in temperature. A field trial was reported, with the addition of 150 ppm of the hydrochloric acid salt of nitrilotriacetamide to an oil well resulted in the almost complete inhibition of salt deposition.
Nitrilotriacetamide was suggested as being useful for stabilizing drilling mud in U.S. Pat. No. 3,544,461 (issued Dec. 1, 1970). Drilling mud is pumped during drilling to remove cuttings and other particulates from around the drill bit. Nitrilotriacetamide was added to sodium chloride saturated drilling mud in order to prevent deposition of solid sodium chloride on filters and equipment. The drilling mud maintains a condition of sodium chloride supersaturation, thereby reducing the tendency of the mud to dissolve salt from the formation contiguous to the well bore. This reduces the amount of mud required for use during the drilling job.
U.S. Pat. No. 5,396,958 (issued Mar. 14, 1995) proposed mud additive compositions and methods for their use to control hole enlargement due to salt bed erosion during drilling, and to assist in solubilization of deposited salt during well operations. The compositions contain sodium or potassium ferrocyanide, and trisodium nitrilotriacetic acid or alkali metal citrates.
Due to the problems associated with downhole salt deposit formation, there exists a need for treatment methods which do not significantly reduce production and are more convenient and cost effective than the currently employed methods.